1. Field of the Invention
The present invention is generally directed to semiconductor lithography tools. In particular, the present invention is directed to a foam core chuck for use in a scanning stage of a lithography system.
2. Background
In a lithography tool, a chuck is commonly used to hold and position an optical piece such as a reticle. Chucks are also used to hold workpieces such as the wafers upon which semiconductor devices are fabricated. As semiconductor devices grow increasingly smaller, the demands on lithography tools increase. Specifically, as chuck positioning tolerances decrease, demands on the lithography tool positioning control systems increase. For example, modem semiconductor geometries require chuck tracking and positioning to be accurate to 10 nanometers or better. In the past, chucks have been made from materials with relatively high thermal expansion coefficients, such as solid silicon carbide. These materials successfully met the less stringent requirements of their era. Conventionally, however, their use is disfavored.
The state of the art is to manufacture the precision portions of lithographic stages, such as wafer and reticle chucks, from ultra low expansion materials. Conventionally, ultra low expansion materials are used in order to keep the thermal strain low. Low thermal strain is desirable for improving the positioning accuracy of the chuck, which holds the reticle or wafer during scanning operations. This is because uncompensated changes in stage dimensions caused by temperature variations increase positioning uncertainty of the object being scanned. Low thermal strain is also desirable because it reduces thermal distortion of reticles and wafers by constraining them with chucks that expand less than the objects being constrained.
Nonetheless, there are several major problems associated with the material properties of ultra low expansion materials conventionally used for chucks in precision lithographic stages. First, they have mediocre specific stiffness. In other words, they are not very stiff for their density. Eigen-frequencies, or resonant frequencies, of these conventional materials are proportional to the square root of their specific stiffness. The lowest chuck resonant frequency, also known as its fundamental frequency, is a limiting factor in selecting the frequency, and thus bandwidth, of the control system for the lithography tool. This is because a control frequency at or above the chuck's fundamental resonant frequency may cause the chuck to vibrate. This compromises the dynamic performance of critical stage components, and adversely affects overall scanning performance and throughput.
Second, ultra low expansion materials have a very low thermal conductivity. As a result, localized heating can occur as heat is not evenly spread through the chuck. Poor heat dissipation limits the amount of heat that can be applied by motors, actuators and the like. Poor heat dissipation also tends to reduce system performance, as heavy cooling and heat shielding components are needed in other moving portions of the stage to prevent heat from affecting the chuck. For example, chucks are magnetically held and positioned within the stage of a modem lithography tool. Flexures are conventionally used to physically isolate the chuck body from the magnets to prevent heat transmitted to the magnets from being transmitted to the chuck body, and to allow for thermal expansion of the magnets. Three flexures are typically used to couple each magnet to the chuck itself. Each chuck typically has six magnets for a total of 18 flexures. Thus, their use also increases the weight of the chuck that needs to be supported and propelled by the positioning and control system.
Third, chucks made from ultra low expansion are very massive. In fact, the mass of the chuck is one of the main factors limiting the throughput of conventional lithography machines. Highly massive chuck blocks require large actuators and motors, which add on to the total moving mass of the chuck, thus aggravating the problem. Indeed, the excessive massiveness of the conventional chuck and its associated parts propagates outwards to the entire stage. The stage thus requires powerful drives, large balance masses and heavy frames to support them. The compounded outcome is that conventional lithography tools often use a stage weighing a couple thousand kilograms to propel and position a reticle weighing roughly 0.5 kilograms. Cost, which increases as a function of weight, is therefore very high.
The path to achieving high throughput at a reasonable cost thus starts with reducing the weight of the chuck. A lighter chuck would enable lighter components across the entire stage, significantly reducing costs and increasing throughput. It would be beneficial to accomplish this without sacrificing control system bandwidth, which is becoming increasingly important as semiconductor device sizes shrink into the nanometer realm. A lighter chuck with high specific stiffness is desired for making critical components of precision lithographic stages.